1. Field of the Invention
The present invention relates to a color image processing apparatus built in a digital color copying machine or the like, and more particularly, it relates to a color image processing apparatus capable of reproducing both characters and photographs included in an original color image well into a color image.
2. Description of the Prior Art
Japanese Unexamined Patent Publication No. 241978/1989, for example, discloses a prior art embodiment of a color image processing apparatus capable of reproducing color graphics that are composed of characters and photographs well into color images.
The exemplary color image processing apparatus set forth in the official gazette processes an image to reproduce original solid black characters into images having no blur with color. For that purpose, the prior art apparatus performs a high resolution processing to reproduce characters with high resolution. In a high resolution process, an uncolored edge in a color image is detected. "Edge" herein is a part where a density is drastically changed. The detected uncolored edge is emphasized and the gradation of a colored portion contiguous to the noncolored edge is leveled down, and a color signal corresponding to the lowered gradation is further suppressed. As a result, a halftone area is processed into multi-leveled gradation, and the solid black characters and drawings are output at the gradation level closer to shadow with higher resolution.
Meanwhile, when image data already processed by the above-mentioned color image processing apparatus is used to practically reproduce a color image on a sheet, four-color toners including yellow, magenta, cyan and black, are usually used to form an image. A digital color copying machine, for example, has generally only one photoconductor drum for image formation, and therefore, image data is divided into four color image data of yellow, magenta, cyan and black, and the color image data are serially processed; that is, an image forming process which includes the steps of forming an electrostatic latent image on the photoconductor drum and the step of developing the latent image into a toner image is performed for one color after another.
In the prior art color image processing apparatus, a filter circuit and a halftone processing circuit are used commonly for processing the four color image data in forming a color image. Hence, even if a color image data read by a scanner or the like is processed so as to make a high resolution image as disclosed in the above-mentioned official gazette, image data of various colors are processed by the common filter circuit and halftone processing circuit in the practical image forming process, and thus, there arises the disadvantage that the high resolution processing performed in the preceding step does not reflect a sufficient effect upon the development of the toner.
More specifically, although the high resolution processing is performed to reproduce characters with high resolution, image data of various colors are processed according to a common filtering and a common halftone processing for toner development, and thus, there arises the disadvantage that a noncolored edge which is specially emphasized is blurred again.